Drive units for generating an oscillating movement are used, for example, in electric toothbrushes or electric razors. For example, Pat. No. DE 28 20 437 A1 discloses an electric toothbrush with an oscillating armature motor. The oscillating armature motor is used for realizing an oscillating rotational movement of a shaft that carries a brush element. The rotational movements are influenced by torsion springs and damping elements such that a desired motion sequence is adjusted.
U.S. Pat. No. 5,613,259 discloses an oscillating appliance in the form of an electric toothbrush. This appliance features a mechanical oscillator that is driven by an electric motor. The electric motor is controlled in dependence on the oscillating frequency of the mechanical oscillator that is determined by means of a sensor, such that the mechanical oscillator remains resonant under a varying load. The mechanical oscillator is realized in the form of a spring-mass system that may comprise a coil spring or a torsion rod. In order to achieve a substantially optimal operation of the electric drive unit for generating an oscillating movement, the drive unit should be excited with a frequency that substantially corresponds or lies close to its resonant frequency. However, the resonant frequency not only changes with the load of the drive unit, but is also detuned due to manufacturing tolerances associated with series production. Although regulating the excitation frequency can compensate for this detuning, such control results in increased manufacturing costs. Moreover, changing the excitation frequency is undesirable in certain applications.